1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device and a semiconductor device, and particularly to a method for manufacturing a semiconductor device that has a metal gate electrode and a semiconductor device obtained based on the method.
2. Description of the Related Art
Metal gate electrodes have been studied as an alternative technique to existing polycrystalline silicon (Poly-Si) gate electrodes. When a metal gate electrode is integrated into an N-channel MOS field effect transistor (NMOSFET) and a P-channel MOS field effect transistor (PMOSFET), a work function control metal having a proper work function needs to be used for the gate electrode of each of the transistors in order to achieve a proper threshold voltage of each of the transistors. However, the work function control metal is not necessarily a low-resistance material. Therefore, forming a gate electrode only by use of the work function control metal is expected to lead to increased gate resistance, which will result in a longer time until the voltage applied to the transistor reaches the threshold voltage of the transistor and hence lower transistor speed.
A method has been proposed to decrease the gate resistance of an entire gate electrode in order to avoid this problem. In this method, a thin work function control metal is deposited over a substrate with the intermediary of a gate insulating film therebetween, and then a low-resistance metal such as tungsten (W) or ruthenium (Ru) is deposited as an upper electrode with a large thickness (refer to e.g. Y. Akasaka, et. al “Impact of Electrode-side Chemical Structure on Electron Mobility in Metal/HfO2 MISFETs with sub-1 nm EOT” Symposium on VLSI Technology Digest of Technical Papers, (USA), 2005, p. 228).